Gases which form hydrates can in particular comprise at least one hydrocarbon selected from methane, ethane, ethylene, propane, propene, n-butane and isobutane, and possibly H.sub.2 S and/or CO.sub.2.
Such hydrates form when water is found in the presence of gas, either in a free state or dissolved in a liquid phase such as a liquid hydrocarbon, and when the temperature reached by the mixture, in particular water, gas and possibly liquid hydrocarbons such as oil, drops below the thermodynamic hydrate formation temperature, that temperature being given for a known composition of gases at a fixed pressure.
Hydrate formation is feared in particular in the gas and oilwell industry where the hydrate formation conditions can be satisfied. In order to reduce the production costs of crude oil and gas, both as regards investment and exploitation, one route, particularly for offshore production, is to reduce or even do away with drying treatments carried out on the crude or on the gas to be transported from the field to the coast and in particular to leave all or part of the water in the fluid to be transported. Offshore treatments are generally carried out on a platform located on the surface near the field, such that the effluent, which is initially hot, can be treated before the thermodynamic hydrate formation conditions are satisfied when seawater cools the effluent.
However in practice, when the thermodynamic conditions required for hydrate formation are satisfied, hydrate agglomeration causes the transport conduits to block through the formation of plugs which prevent the passage of any crude oil or gas.
Hydrate plug formation can cause a production stoppage and thus result in substantial financial losses. Further, restarting the installation, especially when it involves offshore production or transport, can be a long process, as it is difficult to decompose the hydrates which have formed. When the production from an undersea natural gas or crude oil and gas field comprising water reaches the surface of the sea bed and is then transported along the sea bottom, the reduction in the temperature of the effluent produced can mean that the thermodynamic conditions for hydrates to form are satisfied and they form, agglomerate and block the transfer conduits. The sea bottom temperature can, for example, be 3.degree. C. or 4.degree. C.
Favourable conditions for hydrate formation can also be satisfied onshore when conduits are not buried (or are not buried deeply) in the soil, for example when the ambient air temperature is low.
In order to overcome these disadvantages, prior authors have sought products which when added to a fluid can act as inhibitors by reducing the thermodynamic hydrate formation temperature. They are mainly alcohols, such as methanol, or glycols such as mono-, di- or tri-ethylene glycol. This solution is very expensive as the quantity of inhibitors which have to be added can be as high as 10% to 40% of the amount of water and the inhibitors are difficult to recover completely.
Insulation of the transport conduits has also been recommended, to prevent the temperature of the transported fluid from reaching the hydrate formation temperature under the operating conditions. However, this technique is also very expensive.
Further, a variety of non-ionic or anionic surfactant compounds have been tested for their retarding effect on hydrate formation in a fluid comprising a gas, in particular a hydrocarbon, and water. An example can be found in the article by Kuliev et al.: "Surfactants Studied as Hydrate Formation Inhibitors", Gazovoe Delo n.degree. 10, 1972, 17-19, reported in Chemical Abstracts 80, 1974, 98122r.
The use of additives which can modify the hydrate formation mechanism have also been described where, instead of rapidly agglomerating together to form plugs, the hydrates formed disperse in the fluid without agglomerating and without obstructing the conduits. Examples in this regard are the Assignee's European patent application EP-A-0 323 774 which describes the use of non-ionic amphiphilic compounds selected from esters of polyols and carboxylic acids, which may or may not be substituted, and compounds containing an imide function; the Assignee's European patent application EP-A-0 323 775, which describes the use of compounds of the family of fatty acid diethanolamides or fatty acid derivatives; U.S. Pat. No. 4 856 593 which describes the use of surfactants such as organic phosphonates, phosphate esters, phosphonic acids, their salts and their esters, inorganic polyphosphates and their esters, and polyacrylamides and polyacrylates; and European patent application EP-A-0 457 375, which describes the use of anionic surfactants such as alkylarylsulphonic acids and their alkali metal salts.
Amphiphilic compounds obtained by reacting at least one succinic derivative selected from the group formed by polyalkenylsuccinic anhydrides and acids with at least one polyethylene glycol monoether have also been proposed for reducing the tendency of hydrates of natural gas, petroleum gas or other gases to agglomerate (EP-A-0 582 507).